Body panels for automotive vehicles are currently being manufactured using a superplastic forming process applied to certain magnesium-containing aluminum alloy sheet stock. At the present time, the sheet stock is a specially prepared fine grain microstructure, aluminum alloy 5083. AA5083 has a nominal composition, by weight, of about 4 to 5 percent magnesium, 0.3 to 1 percent manganese, a maximum of 0.25 percent chromium, about 0.1 percent copper, up to about 0.3 percent iron, up to about 0.2 percent silicon, and the balance substantially all aluminum. Generally, the alloy is cast into a slab of a suitable thickness and subjected to a homogenizing heat treatment. The slab is then gradually reduced in thickness by a series of hot rolling operations to a strip in the range of twenty to forty millimeters depending somewhat on the goal for the final thickness of the sheet. The strip is then cold rolled, usually in stages with possible interposed anneals, to a final sheet thickness in the range of about one to three or four millimeters. The result of the thermomechanical processing is a coil of smooth surface, aluminum sheet stock, the microstructure of which has been severely strained.
If this aluminum sheet stock is ultimately to be formed into automotive vehicle external body panels, or the like, the smooth surface of the cold rolled sheet is very important. The cold rolled surface must be free of visual defects and that surface quality must be retained in subsequent heating, forming and finishing steps so that the visible surface of the final part is commercially acceptable. In automotive manufacturing such a surface is called a “Class A surface.”
The cold rolled strip is not suitable for a high elongation forming operation. It must be reheated to recrystallize the elongated, strained grains that characterize its microstructure by the nucleation and growth of nearly strain-free grains. The goal of the recrystallizing heat treatment in the case of AA5083 sheet is to produce a very fine grained microstructure characterized by a principal phase of a solid solution of magnesium in aluminum, with well distributed, finely disbursed particles of intermetallic compounds containing minor alloying constituents such as, Al6Mn. The recrystallized grain size in the microstructure is uniformly about ten to fifteen micrometers. Because the dispersed phase is so small the material is sometimes described as “pseudo single phase.” The fine-grained sheet can be heated and superplastically formed into a complex part like an automotive body panel. The sheet can sustain substantial elongation at a suitable strain rate and at a temperature in the range of about 440° C. (825° F.) to about 550° C. (1020° F.).
U.S. Pat. No. 6,253,588 entitled “Quick Plastic Forming of Aluminum Alloy Sheet Metal,” by Rashid et al. and assigned to the assignee of this invention, discloses practices by which the aluminum alloy sheet metal is stretch formed at a suitable forming temperature into automotive body panels and the like. The '588 patent describes practices for forming aluminum alloy sheet metal using a pressurized working fluid such as air. In accordance with this practice, the sheet metal blank is first placed on a pre-bending and heating tool. The heated tool heats the sheet metal blank to its forming temperature and pre-bends it, if desired, for placement on a second tool configured for stretch-forming the heated sheet into a body panel or the like. The heated blank is then clamped at its edges and gas pressure is applied which forces the sheet into the tool cavity to assume the requisite shape of the part. The preparation of the sheet material before forming is critical so that it can sustain the deformation necessary to form the part and retain a commercially acceptable surface finish.
If the sheet metal blank selected for forming has been recrystallized by the coil manufacturer (i.e., supplied in the soft, fully annealed O temper condition), the heating on the pre-heat tool may further the grain growth of its microstructure. Alternatively, if a blank is taken from a cold rolled coil supplied without heat treatment, e.g., in the H18 temper, the metal is not formable because it has experienced a cold rolling reduction of 74% or more as a last processing step. When an un-recrystallized blank is placed on the preheat and pre-bend tool of the Rashid, et al, '588 patent disclosure, the sheet material is recrystallized as it is slowly heated to the panel forming temperature over a period of five to ten minutes. Once the sheet has been recrystallized and reaches a forming temperature, for example, in the range of 825° F. to 845° F. (about 441° C. to 452° C.), it is bent and transferred to a heated forming press in which it is stretch formed into a vehicle body panel or the like.
The prolonged preheating of the sheet metal blank to effect recrystallization of the cold-worked sheet to produce a superplastic formable microstructure has taken five to ten minutes but produced a very formable sheet. Slow recrystallization of the sheet metal on a forming tool has been used in the commercial production of body panels. However, the heating times on the open tools have not been consistent and the heating time has become rate limiting for the overall forming process described in the '588 patent. It is now desired to start with blanks from a cold worked coil and more rapidly heat them to enable a faster rate of production. It is intended that the more rapid heating rate will also produce an even finer recrystallized grain size and greater superplastic ductility.
Accordingly, it is an object of this invention to provide a method of consistently conduction heating a cold-worked, potentially superplastic formable, aluminum alloy sheet so as to quickly convert its highly strained microstructure into a recrystallized fine grained microstructure that is suitable for a superplastic forming operation. At the same time that the sheet is being recrystallized it is being heated to a suitable forming temperature, such as a stretch forming temperature. When the application of the part requires Class A surface quality, that quality is maintained. It is also an object of the invention to provide such a heating method applicable to other cold worked sheet metal alloys that can be recrystallized under static conditions to a highly deformable pseudo single phase material.